I can only comment with regard to the Grizzly version of what you posted, which I purchased probably 7 years ago. Their grit was consistent over the entire surface (no irregularities like I have seen in some other variations).
The concern about bending is valid and if you use a typical jig for chisel or plane iron sharpening -- the style with a wheel - you will roll the plates into a nice gentle cuve (essentially the roller will work just the same on the steel plate as it would a piece of sheet metal). However, you can build a simple jig out of a piece of heavy plate glass and a few pieces of 1/16" brass, aluminum or stainless sheetmetal to hold the diamond plates truly flat, which does allow for precise sharpening. I can post a photo of such if anyone is interested.
What I can not say, is whether today's version is of comparable quality to what I purchased 7 years ago... and Harbor Freight quality is often at the bottom of the barrel (they may carry the same merchandise as found elsewhere, but HF's is more likely to contain seconds and discards that some of their higher-priced competitors would have passed on).